Ultrasonic signals can be used to provide motion detection of objects near devices. Such motion detection can then be used for gesture recognition purposes, or any other motion triggered procedure. In particular, such motion detection can be used to activate a function or application of a device or to bring a device in or out of a low-power mode.
The Doppler Effect can be used to detect movement of an object near the detector, by determining the frequency shift of the echoes from a transmitted continuous wave. Usually ultrasonic signals, for example above 20 kHz, are used to avoid interfering with everyday use of the device. Alternatively, a short pulse is transmitted and the time delay between transmission and receipt of an echo pulse is used to determine the distance of an object from the device during that particular time period. The distance determined during different time periods can then be compared to determine if the object is moving or not.
However, these techniques suffer from false detections of object motion due to acoustic sources emitting in the ultrasonic range, for example metallized or metallic objects rustling or jangling. Furthermore, if another motion detection device is being used nearby, the first device may detect and misinterpret the ultrasonic signals being emitted from the nearby device, again resulting in a false detection of object motion. The frequency shift signal obtained using Doppler methods conveys only information about velocity so does not distinguish objects at different distances, and therefore does not allow any control over the range of distances over which the detector responds. Methods relying on an estimate of distance may be confused by simultaneous reflections from multiple different moving objects or from different areas on the same moving object. Systems using Doppler methods also require relatively complex signal processing, and involve significant power consumption, making them unsuitable for continuous use.